Optical modulators, optical switches, and others are used in optical communication represented by, e.g., high-capacity transmission in a metropolitan area network and an intercontinental or intracontinental backbone network and optical interconnection applied to servers and routers, and the like. Of the optical devices, small and high-speed ones are needed in terms of data formation and signal processing. As one of optical devices meeting the needs, a waveguide type optical device using the electrooptic effect has been developed in recent years. See, e.g., JP2007-212787A.
As a waveguide type optical device related to the present application, a Mach-Zehnder optical modulator which is composed of a reversed-ridge waveguide obtained by stacking layers of lanthanum-doped lead zirconate titanate (PLZT) on indium tin oxide (ITO) patterned on a sapphire substrate is disclosed in Document 1 below.
Document 1: “JOURNAL OF LIGHTWAVE TECHNOLOGY”, VOL. 18, NO. 6, JUNE 2000, pp. 807-812 (see, in particular, FIGS. 1, 3, and 8)
A light beam in the optical modulator is guided through a PLZT portion without ITO and is branched into two parallel waveguides through a Y branching waveguide, and light beams propagate through the two parallel waveguides. Next, the light beams are multiplexed by the other Y branching waveguide, and the multiplexed light beam propagates through one waveguide. In such an optical modulator, an electrode is formed in each of an inner area between two parallel waveguides and two outer areas between which the two waveguides are sandwiched. To operate the modulator, a voltage of 0 V is applied to the electrode in the inner area, a voltage of (Vb÷Vm) is applied to the electrode in one of the outer areas, and a voltage of (Vb−Vm) is applied to the electrode in the other outer area.
However, in each ridge waveguide illustrated in Document 1, a core layer is made of PLZT with a high dielectric constant, a top core of the core layer is formed over the entire surface of the device with respect to a bottom core of the core layer which transmits light. In the case of such a core structure, a pair of top and bottom electrodes formed to sandwich the whole region of a top core increases parasitic capacitance. In order to prevent an increase in parasitic capacitance, the core was a structure in which electrodes are positioned above the waveguide so that the waveguide is horizontally sandwiched between the electrodes to apply a voltage, therefore the structure makes it difficult to apply an efficient electric field to the waveguides and their surroundings.
As described above, in a waveguide type optical device using the ridge waveguide described in Document 1, a core material with a high dielectric constant is formed over the entire surface of the device. If electrodes are vertically formed to sandwich cores, parasitic capacitance cannot be reduced. Accordingly, the waveguide type optical device suffers from the problem of the difficulty in achieving a higher signal transmission speed.
Additionally, the electrodes are formed above the waveguides away from a core layer, as described above. An efficient electric field cannot be applied to a waveguide section, thus resulting in the problem in which it is difficult to achieve a lower voltage or lower power.